This invention relates to a phase controlled oscillator, and more particularly an improvement of a phase controlled oscillator in which the frequency of a voltage controlled oscillator is stabilized by an analogue phase locked synchronizing loop.
A prior art phase controlled oscillator is disclosed in U.S. Pat. No. 4,039,966 issued on Aug. 2, 1977, for example, in which the output of a voltage controlled oscillator whose phase is controlled by a varactor diode (trade name) is applied to one input of a comparator with the other input supplied with the output of a reference oscillator. The output of the phase comparator is applied to a loop amplifier and its output signal is supplied to a control input terminal of a voltage controlled oscillator.
With this construction, the loop amplifier operates as a sweeping oscillator until the loop reaches a synchronous state, so that the voltage of the varactor diode of the voltage controlled oscillator is scanned to be controlled in a pull-in range. As the synchronous state is reached the phase of the input signal to the amplifier becomes opposite to that of the output signal so that the amplifier stops to oscillate and acts as a loop amplifier. The phase locking function is well known in the art and disclosed, for example, in a text book of F. M. Gardner, "Phaselock Techniques" published by John Willy and Son, 1979.
Although this circuit can start automatically and can operate as an oscillation circuit operating stably, the hold-in range of the phase locked loop is narrower than a frequency changing range of the voltage controlled oscillator so that pull-in is possible only in a portion of the frequency changing range of the voltage controlled oscillator whereby the characteristic of the amplifier while it acts as a sweeping oscillator would be limited and it can not self-start under a condition other than the condition described above.
Where a plurality of channels are used as in a transponder used in a communication satelite, for example, the frequency range is extremely wide. For example, a reference frequency of 2 GHz requires a range of 100 MHz. However, the frequency range of the oscillator of the type referred to above is about 20 MHz, while the loop band or the locking range is at most 100 KHz. Thus in terms of the pull-in range or capture range the frequency range is about 1 to 2 GHz. Accordingly, the phase locked loop can not operate stably, for the reason described above.